With the recent progress in portable telephony, there is a growing demand for small but stable crystal oscillators. In particular, stable high-frequency oscillation and low power dissipation are greatly to be desired. FIG. 6 shows a conventional ECL (Emitter Coupled Logic) oscillation circuit. This ECL oscillation circuit is made up of: the ECL circuit 20 having non-inverting and inverting input terminals 21 and 22 and non-inverted and inverted output terminals 24 and 23; a frequency control capacitor C11; a crystal oscillator X'tal; a resistor R11; a capacitor C12; and pull-up resistors R12 and R13.
The ECL circuit 20 has its non-inverted output terminal 24 and non-inverting input terminal 21 interconnected via a series connection of the frequency control capacitor C11 and the crystal oscillator X'tal. The non-inverting input terminal 21 is connected via the resistor R11 to the inverting input terminal 22, which is grounded via the capacitor C12. The non-inverted and inverted output terminals 24 and 23 are connected to their power supplies via the pull-up resistors R12 and R13, respectively. The illustrated circuit is configured to provide its output via the non-inveted output terminal 24.
The conventional ECL oscillation circuit of FIG. 6 oscillates with the series connection of the frequency control capacitor C11 and the crystal oscillator X'tal inserted between the non-inverted output terminal 24 and the non-inverting input terminal 21 of the ECL circuit 20. In this instance, the delay between the non-inverted output terminal 24 and non-inverting input terminal 21 serves as an effective inductor, which combines with the capacitance of the series connection of the frequency control capacitor C11 and the crystal oscillator X'tal, thereby causing oscillation.
In the conventional circuit of FIG. 6, the delay between the non-inverted output terminal 24 and non-inverting input terminal 21 of the ECL circuit 20 serves as an effective inductor for oscillation. But the delay time between the non-inverted output terminal 24 and the non-inverting input terminal 21 is likely to vary greatly from one ECL circuit to another even if they are mass-produced with single design. Hence, the oscillation conditions may sometimes become inadequate according to the particular ECL circuit used, resulting in impaired stability of oscillation.